var a = prompt("请输入圆的半径");
var p = Math.PI;



if (!isNaN (a)){
    var b = 2* a * p;
    var c = a * a * p; //math.pow(p,2);表示p的2次幂，等于?;

    document.querySelector("#a").value = a;
    document.querySelector("#b").value = b;
    document.querySelector("#c").value = c;
}else{
    alert("请输入正常数字")
}

